The latest Android Lollipop source code was released on 3rd November this year. So the official builds will be released very soon but not yet available for most of the devices. But xda senior member mo976 has ported the Lollipop CyanogenMod 12 for Google Nexus 5. CyanogenMod is based on AOSP and CyanogenMod 12 will be the name for Android Lollipop version of CyanogenMod.

WARNING:Installing custom ROM requires a custom recovery to be installed on your device, which may VOID your warranty and may delete all your data. Before proceeding further you may consider backup your all data. Please visit the Disclaimer page below to agree with the risks involved and proceed before flashing this ROM to your phone: ROM Disclaimer link

Because this is an un-official build this is not hosted on Cyanogenmod official website and will get the updates only when the developer will release one.

WARNING:The Nexus 5 has no built in EFS backup partitions. EFS contains things like your MEID or IMEI… your data profiles.. radio settings. The EFS cannot be replaced… it is unique to your phone only… backing up your OWN EFS is highly recommended. I will not be responsible for the consequences if you will not go for the EFS backup. EFS backup link